Third generation partnership project 3GPP and 3GPP2 are considering long term evolution LTE for radio interface and network architecture.
There is an ever-increasing demand on wireless operators to provide better quality voice and high-speed data services. As a result, wireless communication systems that enable higher data rates and higher capacities are a pressing need.
To achieve this, it is becoming increasingly popular to use multi-antenna systems in wireless communications networks to obtain advantages of increased channel capacity, spectrum efficiency, system throughputs, peak data rates and/or link reliability. Such multi-antenna systems are generically referred to as multiple-input-multiple-output (MIMO) systems but may also include multiple-input-single-output (MISO) and or single-input-multiple-output (SIMO) configurations.
Efficient signaling is essential to evolved universal terrestrial radio access (E-UTRA). A low overhead control signaling scheme can improve MIMO link performance, system capacity, system throughputs, information data rates and increased spectrum efficiency.
MIMO systems promise high spectral efficiency and have been proposed in many wireless communication standards. A lot of research is also currently underway on precoding for spatially multiplexed or space-time coded MIMO systems. Precoding is a technique used to provide increased array and/or diversity gains.
Precoding information needs to be communicated from a transmitter, (e.g., a base station), to receiver, (e.g., a wireless transmit/receive unit (WTRU)), to avoid a channel mismatch between transmitting and receiving signals. This is particularly important for MIMO data demodulation when precoding is used. When a receiver uses incorrect channel responses for data detection, significant performance degradation can occur.
Generally, precoding information may be communicated using explicit control signaling, particularly when the transmitter and receiver are restricted to the use of limited sets of antenna weights and coefficients for precoding. The limited sets of antenna weights and coefficients are sometimes referred to as precoding codebook. Explicit signaling to communicate precoding information from a transmitter to a receiver may incur large signaling overhead, particularly for a large size codebook. This signaling overhead is magnified manifold when frequency selective precoding is used.
Precoding matrix or antenna weights validation and verification is used to avoid effective channel mismatch between a transmitter and a receiver. An effective channel between a base station and a mobile handset is a channel that experiences MIMO precoding effect, and is the multiplication of channel matrix H and precoding matrix V used at an evolved Node-B (eNodeB) or a transmitter. A mismatch of the effective channel between the transmitter and the receiver causes severe performance degradation for MIMO communication systems.
FIG. 1A shows a precoding matrix or antenna weights signaling scheme. In a scheme as shown in FIG. 1, a wireless transmit/receive unit (WTRU) 111 feeds back precoding matrix indices (PMIs) or antenna weights to a base station or an eNodeB 113. Suppose that the WTRU feeds back PMI_j (having Y bits) 115 to an eNodeB. To inform the WTRU of current precoding matrix used at the eNodeB, the eNodeB sends a validation message PMI_k (Y bits) 117 to the WTRU. In case of feedback error or override, PMI_j is not equal to PMI_k. In case of no feedback error and no eNodeB override, PMI_j=PMI_k. The validation message can be sent in several forms, for example via control signaling or via reference signal.
In some systems such as Wideband Code Division Multiple Access WCDMA, there is only one PMI needed to be signaled to receiver from transmitter and vice versa. The signals are transmitted in time domain using spreading code. Signaling the exact single PMI (Y bits) to receiver does not incur too much overhead as long as the value of Y is reasonable. However in some systems such as Orthogonal frequency-division multiplexing OFDM systems, where frequency domain is additional to time domain, there may be multiple PMIs needed to be fed back from the WTRU and sent from the eNodeB for validation to support frequency selective precoding. Frequency selective precoding performs MIMO precoding per sub-band within system bandwidth. The entire system bandwidth can be divided into several sub-bands. Each sub-band consists of one or several sub-carriers. One precoding matrix is used to precode transmitted data per sub-band. In an extreme case precoding can be performed per sub-carrier if a sub-band consists of only a sub-carrier. If multiple PMIs are needed to be signaled to receiver (WTRU), then the signaling overhead could be significant. For example if there are Z PMIs to be signaled and each PMI has Y bits, then the total overhead is Z×Y bits. If Z or Y itself are large, the signaling overhead is significant.
The terminology for precoding matrix and precoding vector is interchangeable and depends upon the number of data streams to be precoded.
Each PMI is represented by L bits, wherein the value of L depends upon MIMO configurations and codebook sizes and number of data streams to be supported. WTRUs are assigned resources for communications. A resource block (RB) consists of M subcarriers, for example, M can take the value twelve (12). A resource block group (RBG) or sub-band consists of N resource blocks (N_RB), for example, N_RB=2, 4, 5, 6, 10, 25 or entire bandwidth. A system bandwidth can have one or more RBGs or sub-bands depending on the size of the bandwidth and the value of N_RB per RBG. For example, the number of RBGs per system bandwidth, N_RBG, can be one, two, four, ten, twenty and fifty. In general, the terminology RBG and sub-band is interchangeable.
The WTRU feeds back one PMI for each RBG that is configured for or selected by the WTRU for reporting. Among N_RBG RBGs for a given bandwidth, N RBGs, where ‘N≤N_RBG’ can be configured for or selected by a WTRU. If ‘N’ RBGs are configured for or selected by a WTRU for reporting precoding information, the WTRU feeds back ‘N’ PMIs to the eNodeB. The eNodeB sends the precoding validation message comprising ‘N’ PMIs back to the WTRU.
To inform the WTRU of current PMIs used at the eNodeB, the eNodeB sends ‘N’ PMIs back to the WTRU. The total number of bits that the eNodeB sends to the WTRU per PMI validation message is ‘N_PMI×N’ bits.
Table 1A shows the number of bits for PMI validation message assuming N_PMI=5 bits. The numbers are summarized for 5, 10 and 20 MHz system bandwidth. The second row is N_RB, the number of RBs per RBG. For example, the N_RB ranges from 2 to 100 for 20 MHz. The third row is N_RBG per system bandwidth, i.e., number of RBGs per system bandwidth of 5, 10 or 20 MHz, and the value of N_RBG ranges from one to fifty 50. The fourth row is the total number of bits for PMI validation signaling per validation message or grant channel.
TABLE 1A5 MHz10 MHz20 MHz300 (subcarriers)600 (subcarriers)1200 (subcarriers)N_RB per RBG2510252510255025102550100N_RBG per band135312510521502010421Total #of bits for PMI652515512550251052501005020105signaling per validationmessageAssume 12 subcarriers per RB.N_RB: Number of resource blocks.N_RBG: Number of frequency blocks for pre-coding control unit to which assigned RBs belong.N_PMI: Number of bits to represent a PMI.Maximum total number of bits per PMI validation message = N_RBG × N_PMI.
This precoding matrix/matrices or antenna weights validation, hereinafter called “precoding information validation” or “PMI validation”, may require up to 250 bits or more per validation message. Hence, this scheme is inefficient.
Therefore, it would be desirable to provide a method and apparatus to reduce the signaling overhead for PMI validation.